Typically, distillation unit operations are conducted in countercurrent multi-tray towers in which the liquid flows across the trays and down the column by way of downcomers, while the gas passes upward through the trays, which may contain contact plates, bubble caps, sieves or similar contact devices. Mass transfer occurs in the liquid-gas mixtures near the bottom of each tray. Phase separation, necessary to secure overall countercurrent flow of gas and liquid, takes place by gravity in the spaces between the trays.
During the flow of the vapor and the liquid through the unit, liquid particles or droplets become entrained in the vapor phase of the upwardly flowing gaseous stream. The entrained liquid is difficult to separate from the flowing stream, and various techniques have been employed to provide phase separation. Various mechanisms may be exploited in the separation. Micro-droplets may be coalesced to form larger drops and liquid particles may be separated by centrifugal force, gravity, or impingement on solid surfaces. Demister screens and flow deflectors of varied design have proven effective in phase separation. Many of these devices, however, are expensive to fabricate and operate, and create excessive pressure drops.
U.S. Pat. No. 4,361,469 (Trutna) teaches co-current distillation components employing a multi-level array of rectangular horizontal channels (or U-shaped troughs) adapted for liquid separation, collection and distribution. The liquid collected in the channels is discharged to a downcomer that delivers the liquid to the tray of a lower stage. The de-entrainment devices may be installed above one or more distillation trays, as required by the particular application. In arrangements of this type, it is expected that most of the liquid swept up between one set of channels will continue to be entrained upward by the gas. The main liquid collection mechanism seems to be eddy flows past the upper edges of the channels. While this method exhibits a low pressure drop, the relative amount of recovered liquid particles is lower than desired. This deficiency is believed to be related to the relatively unrestricted flow path of the vapor stream with entrained particles as it passes through the array of channels.